1. Technical Field
This disclosure relates generally to implantable medical devices, and more particularly to monitoring power consumption.
2. Background Information
There have been many improvements over the last several decades in medical treatments for disorders of the nervous system, such as epilepsy and other motor disorders, and abnormal neural discharge disorders. One of the more recently available treatments involves the application of an electrical signal to reduce various symptoms or effects caused by such neural disorders. For example, electrical signals have been successfully applied at strategic locations in the human body to provide various benefits, including reducing occurrences of seizures and/or improving or ameliorating other conditions. A particular example of such a treatment regimen involves applying an electrical signal to the vagus nerve of the human body to reduce or eliminate epileptic seizures, as described in U.S. Pat. No. 4,702,254 to Dr. Jacob Zabara, which is hereby incorporated by reference in its entirety in this specification. Electrical stimulation of the vagus nerve may be provided by implanting an electrical device underneath the skin of a patient and electrically stimulating tissue, organ(s) or nerves of the patient. The system may operate without a detection system if the patient has been diagnosed with epilepsy, and may periodically apply a prophylactic series of electrical pulses to the vagus (or other cranial) nerve intermittently throughout the day, or over another predetermined time interval.
Typically, implantable medical devices (IMDs) involving the delivery of electrical pulses to, or sensing electrical activity of, body tissues, such as pacemakers (heart tissue) and vagus nerve stimulators (nerve tissue), comprise a pulse generator for generating the electrical pulses and a lead assembly coupled at its proximal end to the pulse generator terminals and at its distal end to one or more electrodes in contact with the body tissue to be stimulated. One of the key components of such IMDs is the power supply (e.g., a battery), which may or may not be rechargeable. In many cases surgery is required to replace an exhausted battery. Even rechargeable batteries eventually may need replacement. To provide adequate warning of impending depletion of the battery and subsequent degradation of the operation of the IMD, various warning signals or indicators may be established and monitored.
Generally, battery-powered IMDs base warning signals or indicators on battery voltage and/or battery impedance measurements. One problem associated with these methodologies is that, for many battery chemistries, these measured battery characteristics do not have monotonically-changing values with respect to remaining charge. For example, lithium/carbon monofluoride (Li/CFx) cells commonly used in neurostimulators and other IMDs have a relatively flat voltage discharge curve for the majority of their charge life, and present status of the battery cannot be accurately and unambiguously determined from a measured battery characteristic.
More specifically, in LiCFx batteries, the battery voltage remains relatively constant for approximately 90% of its useful life and then reaches a point where the battery changes from a linear region of approximately zero slope to an approximately linear or downwardly curving region of negative slope. Thus, during the last 10% of battery life (when battery voltage versus battery depletion enters the second range), an added term in the projection equation that incorporates battery voltage may improve the accuracy of the projection to the battery's depletion.
Another problem associated with impedance-based methodologies is the variability of current consumption for a specific device's programmed therapy or circuitry. This variability, combined with the uncertainty of the battery's present status prior to depletion, hinders reliable estimation of the anticipated time until reaching the end of the battery's useful life. For scheduling purposes, it is desirable to have a constantly available and reliable estimate over all therapeutic parameter ranges and operation settings of the time until the device will reach the end of its useful life, and provide an indication when that time reaches a specific value or range.
The present disclosure is directed to overcoming, or at least reducing, the effects of, one or more of the problems set forth above.